To build a 3d-printer you don't need to print a single part. But the goal of the Reprap project is to explore and develop maximally self-replicating machines. Some of the results are useful, some maybe not so much.
One of my personal interests is in making it possible to replicate an full makerspace/fablab by visiting an existing one. Bootstrap one or two digital fabrication machine by borrowing access, then build everything else with those in your own lab. This benefits from high degree of self- or co-replicability.
The idea can be applied outside 3d-printers as well. Here is a CNC machine you can CNC (including motion/drivetrain) https://github.com/fellesverkstedet/fabricatable-machines/bl...
One of my personal interests is in making it possible to replicate an full makerspace/fablab by visiting an existing one. Bootstrap one or two digital fabrication machine by borrowing access, then build everything else with those in your own lab. This benefits from high degree of self- or co-replicability.